[Paper Review] The GRAVITY Young Stellar Object survey IV. The CO overtone emission in 51 Oph at sub-au scales

Collaboration Gravity, M. Koutoulaki+70 more|arXiv (Cornell University)|Nov 11, 2020

Astrophysics and Star Formation Studies46 references2 citations

TL;DR

Using high-resolution interferometry with the GRAVITY instrument on the VLT, this study spatially resolves CO overtone emission in the innermost disc of the Herbig Ae/Be star 51 Oph. The data reveal a compact, hot (1900–2800 K), dense (0.9–9×10²¹ cm⁻²) CO-emitting region at 0.10±0.02 au from the star, located well within the dust sublimation radius and co-located with the dusty disc, challenging conventional disc models that exclude CO in dust-free inner regions.

ABSTRACT

51 Oph is a Herbig Ae/Be star that exhibits strong near-infrared CO ro-vibrational emission at 2.3 micron, most likely originating in the innermost regions of a circumstellar disc. We aim to obtain the physical and geometrical properties of the system by spatially resolving the circumstellar environment of the inner gaseous disc. We used the second-generation VLTI/GRAVITY to spatially resolve the continuum and the CO overtone emission. We obtained data over 12 baselines with the auxiliary telescopes and derive visibilities, and the differential and closure phases as a function of wavelength. We used a simple LTE ring model of the CO emission to reproduce the spectrum and CO line displacements. Our interferometric data show that the star is marginally resolved at our spatial resolution, with a radius of 10.58+-2.65 Rsun.The K-band continuum emission from the disc is inclined by 63+-1 deg, with a position angle of 116+-1 deg, and 4+-0.8 mas (0.5+-0.1 au) across. The visibilities increase within the CO line emission, indicating that the CO is emitted within the dust-sublimation radius.By modelling the CO bandhead spectrum, we derive that the CO is emitted from a hot (T=1900-2800 K) and dense (NCO=(0.9-9)x10^21 cm^-2) gas. The analysis of the CO line displacement with respect to the continuum allows us to infer that the CO is emitted from a region 0.10+-0.02 au across, well within the dust-sublimation radius. The inclination and position angle of the CO line emitting region is consistent with that of the dusty disc. Our spatially resolved interferometric observations confirm the CO ro-vibrational emission within the dust-free region of the inner disc. Conventional disc models exclude the presence of CO in the dust-depleted regions of Herbig AeBe stars. Ad hoc models of the innermost disc regions, that can compute the properties of the dust-free inner disc, are therefore required.

Motivation & Objective

• To determine the physical and geometrical structure of the inner gaseous disc in 51 Oph using spatially resolved interferometric data.
• To investigate the origin and distribution of CO ro-vibrational emission in the dust-free inner disc region.
• To test whether conventional disc models can account for CO emission in regions devoid of dust.
• To constrain the temperature, column density, size, inclination, position angle, and kinematics of the CO-emitting gas.

Proposed method

• Used the GRAVITY instrument on the Very Large Telescope Interferometer to obtain high-spectral-resolution K-band interferometric data.
• Measured visibilities, differential phases, and closure phases across 12 baselines to reconstruct spatial and spectral information.
• Applied a local thermal equilibrium ring model to fit the CO bandhead spectrum and phase shifts relative to the continuum.
• Used synthetic intensity maps of CO emission to reproduce observed displacements and infer the size, inclination, position angle, and rotational velocity of the emitting region.
• Constrained the physical parameters (T, NCO) by fitting the observed CO spectrum under the assumption of optically thick J-components.
• Compared the geometry and kinematics of the CO-emitting region with those of the dusty disc to assess alignment.

Experimental results

Research questions

• RQ1 Where is the CO overtone emission spatially located relative to the dust sublimation radius in 51 Oph?
• RQ2 What are the physical conditions (temperature, column density) of the CO-emitting gas in the inner disc?
• RQ3 Is the CO-emitting region aligned with the dusty disc in terms of inclination and position angle?
• RQ4 What is the size and kinematic structure of the CO-emitting region?
• RQ5 Can standard disc models explain the presence of CO in the dust-free inner disc?

Key findings

• The CO emission originates from a region 0.10±0.02 au in radius, located well within the dust sublimation radius.
• The CO-emitting gas is hot (1900–2800 K) and dense (0.9–9×10²¹ cm⁻²), consistent with previous studies.
• The inclination of the CO-emitting region is 63°±1° and position angle is 116°±1°, matching that of the dusty disc.
• The CO emission is more compact than the K-band continuum, with visibility increases within CO bandheads indicating a smaller emitting region.
• The CO line displacements relative to the continuum are best reproduced by a ring model at 0.1 au, confirming its compact size.
• The alignment between the CO and dusty disc suggests no significant misalignment, supporting a co-eval origin of the gaseous and dusty components.